The broad objective of this project is to elucidate the processes by which the cardiovascular system responds to the stress of physical exercise. Utilization of techniques recently developed by the principal investigator will permit accurate, continuous measurement of left ventricular internal diameter and pressure, as well as pressures and flows in the major vessels, using chronically implanted electronic devices in dogs exercising on a treadmill. These data will be used to evaluate the relevance of hemodynamic mechanisms and alterations in myocardial contractile strength to left ventricular performance during exercise. Specific investigations will be made comparing the hemodynamics of mild versus vigorous exercise, and studying the influence of autonomic nervous system activity on the exercise response, the effect of inotropic agents on the exercise response, the effect of pathological cardiac lesions on the exercise response, and the effect of conditioning and deconditioning on the exercise response.